1. Field of the Invention
The present invention relates to a driver circuit, specifically a small signal swing driver, which generates a reduced amount of switching noise and suppresses voltage transients on the line, such as reflections, appearing at the output of the driver.
2. Description of the Prior Art
Although integrated circuits themselves along with their interconnect topology have become increasingly dense, the need nonetheless exists to route certain digital signals, via interconnect lines, between circuitry located on an integrated circuit chip and off chip circuitry.
In these instances, a digital signal is first applied to a line driver which, in response thereto, applies electrical current to an interconnect line in order to rapidly charge up the line capacitance and thereby cause the digital signal to propagate down the line. Each interconnect line possesses a certain characteristic impedance and hence behaves as a transmission line. Consequently, if the output impedance of the driver located at the near end of the line and a load located at the far end of the line are not equal to the characteristic impedance of the line, reflections result.
In particular, whenever the output of the driver changes state from a logical zero (often a DC potential near ground) to a logical one (often a higher DC voltage), a rising output voltage transition propagates, through the line, from the driver to the far end of the line. If an impedance mis-match exists at the far end resulting from, for example, an open (unterminated) circuit, then, a portion of this transition reflects off the mis-match and propagates back to the near end of the line, i.e. toward the output of the driver. After a time interval--governed by the length of the line and usually on the order of a few nano-seconds for a short line (10 centimeters or less)--has elapsed, the reflection appears at the driver output and combines with the driver output voltage. Similarly, reflections can also be generated by impedance mis-matches occurring at the near end of the line. Near end reflections propagate down the line and cause further reflections at the far end. In any event, depending upon the amplitude of the driver output voltage, the combination of a non-zero driver output voltage and the reflected voltage appearing at the near end of the line may produce a voltage transient which, at the output of the driver, exceeds the voltage produced by the driver itself. The resulting combined voltage appears across the driver output transistors. As a result, this voltage, if it possesses a sufficiently large transient peak amplitude, may destroy these transistors.
Owing to impedance mismatches occurring at both ends of the line, reflections will continue propagating between both ends of the line until the reflected energy has been sufficiently absorbed by resistive loss occurring in both the line and the driver. Hence, the voltage transient will appear as an exponential decaying damped sinusoid.
Further problems arise since inputs to other gates are connected to the interconnect line. Specifically, these gates might sense an undesired high level input signal whenever a transient resulting from a reflection occurs. As a result, these gates might generate erroneous output values which, in turn, might cause improper system operation.
Thus, it is imperative to eliminate reflections whenever possible in the design of digital systems. Although the solution--match all driver output and load impedances to the line--is simple in theory, in practice the complexity of interconnect wiring and/or the diversity of the circuitry connected thereto renders this solution extremely difficult to achieve.
These problems worsen whenever small input swing logic, which provides shortened transition times, decreased switching noise and increased switching speed, is employed. With this logic, the voltage difference between the different logic levels is on the order of one volt or less with the logic levels swinging from, for example, +0.5 volts for a logical zero to +1.5 volts for a logical one. Since the noise margin for such logic is far less than that existing in many other types of digital logic, a small amount of noise, e.g. a voltage transient, on the line, resulting from, for example, reflections, can readily produce a false input condition for a gate connected to that line.
In addition, noise can be coupled into a quiet interconnect line from gates or other lines situated close by but not connected to the quiet line. In this situation, very large and narrow current spikes are often generated by high switching rates (dv/dt on the order of several volts per nano-second) present in nearby gates and/or lines. These spikes, in turn, generate magnetic fields that induce transient voltage spikes, here switching noise, into the quiet interconnect line. These spikes, if sufficiently large, can also cause a false input condition to occur at the input to a small signal swing gate connected to the quiet line.
Furthermore, switching noise can also be coupled through ground paths to all gates situated in a particular circuit or system. Consequently, this noise will appear not only on those gates being switched but also on those that are not. Unfortunately, as switching speeds increase, so does the amount and intensity of switching noise.
Various solutions exist in the art for preventing transients from affecting the output stage of a driver. For example, one solution is described in U.S. Pat. No. 3,979,607 (issued to H. R. Beelitz et al on Sept. 1, 1976--the '607 patent). Here, a driver incorporates a push-pull output stage and a shunting transistor. The base and emitter of the shunting transistor are connected in parallel with the base and emitter, respectively, of the driver pull-down output transistor. In addition, the collector of the shunting transistor is coupled to the base of the driver pull-up output transistor. Whenever the shunting transistor conducts, it limits the amount of base drive available to the pull-down transistor, thereby limiting its collector current. Unfortunately, this arrangement is only useful to suppresses current transients that occur within the driver itself, i.e. excessive currents commonly known as "spike through" currents, that flow between the pull-up and pull-down output transistors whenever both transistors simultaneously conduct. This driver does not have the capability to suppress transients appearing on the line due to conditions, e.g. reflections, that are external to the driver. U.S. Pat. No. 4,031,414 (issued to T. C. Giles on June 21, 1977) shows an arrangement for a rapid rise, short-duration current pulse generator. This arrangement includes a voltage limiting circuit which prevents excessive voltage levels from being applied to an internal current source during the time interval occurring between successive output current pulses. This arrangement, like that shown in U.S. Pat. No. 3,979,607, only prevents transients from occurring within the driver itself and does not suppress externally induced transients which appear on the line.
Therefore, a need exists in the art for a small signal swing logic line driver that generates a reduced amount of switching noise and also suppresses voltage transients that occur on the line which result from conditions external to the driver.